Recent compelling evidence demonstrates that sclerostin - the product of the Sost gene exclusively expressed by osteocytes in bone - antagonizes the pro-osteoblastogenic actions of Wnts and BMPs;providing a long-sought molecular means by which osteocytes regulate bone formation. Work leading to this application by the PI and collaborators demonstrated that chronic elevation of parathyroid hormone (PTH) potently decreases Sost/sclerostin expression in osteocytes in vivo and in vitro, suggesting a novel mechanism for PTH-dependent osteoblastogenesis mediated by osteocytes. Mechanical loading also increases osteoblast number;and a potential mediator of this anabolic effect is PTH related peptide (PTHrP), as its expression is increased by mechanical stimulation. Notably, Sost expression in inhibited by mechanical stimuli in vitro and by bone loading in vivo. Moreover, transgenic mice overexpressing a constitutively active PTH 1 receptor (PTHR1) exclusively in osteocytes (DMP1-caPTHR1) exhibit decreased Sost expression and a remarkable increase in bone mass. Based on these lines of evidence, it is hypothesized that activation of PTHR1 signaling in osteocytes leads to a rapid and direct inhibition of Sost gene expression, which, in turn, is responsible for increased bone formation in response to systemic elevation of PTH as well as to bone loading through local increase in PTHrP. This hypothesis will be tested by a combination of in vitro studies using osteocytes generated in vitro and authentic osteocytes, and in vivo approaches using transgenic and knock out mice. Studies in Aim 1 will elucidate the signaling pathways responsible by the rapid inhibition of Sost expression by PTH and PTHrP. In Aim 2, it will be determined whether suppression of Sost expression in vitro by mechanical stimulation induced by stretching or oscillating fluid flow requires PTHR1 signaling and PTHrP;and whether the decreased sclerostin expression by bone loading is spatially related to increased PTHrP and increased bone formation using the model of ulna loading in mice. In Aim 3, the consequences of PTHR1 activation or deletion in osteocytes in vivo will be established by complementary transgenic and knock out approaches. It will be also examined whether the osteogenic response induced by PTH elevation or loading, or the high bone mass phenotype of DMP1- caPTHRI mice are reversed, or at least ameliorated, by Sost overexpression or by blocking the Wnt signaling pathway. Furthermore, it will be established whether the reduction in Sost expression and the osteoblastogenic response to PTH or mechanical loading are abrogated in mice in which the PTHR1 is knocked out specifically in osteocytes (DMP1-10kb-Cre/PTHR1 mice). These studies will advance understanding of the control of bone formation by osteocytes and will elucidate the contribution of these cells to the osteoblastogenic actions of PTH, PTHrP, and mechanical stimuli. We expect that this work will provide opportunities for the development of novel therapeutic approaches leading to bone anabolism through actions on osteocytes.